Keratoconus is a degenerative non-inflammatory disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve. Keratoconus can cause substantial distortion of vision, with multiple images, streaking and sensitivity to light all often reported by the patient. Keratoconus is the most common dystrophy of the cornea, affecting around one person in a thousand, and it seems to occur in all ethnic groups worldwide, although for some groups the prevalence of keratoconus is greater than others. It is typically diagnosed in the patient's adolescent years and attains its most severe state in the twenties and thirties.
Keratoconus is a little-understood disease with an uncertain cause, and its progression following diagnosis is unpredictable. If afflicting both eyes, the deterioration in vision can affect the patient's ability to drive a car or read normal print. It does not however lead to blindness, and in most cases, corrective lenses are effective enough to allow the patient to continue to drive legally and likewise function normally. Further progression of the disease may lead to a need for surgery. Despite its uncertainties, keratoconus can be successfully managed with a variety of clinical and surgical techniques, and often with little or no impairment to the patient's quality of life.
Pellucid marginal degeneration (PMD)—is a rare condition whereby the lower cornea becomes thinner and the optic surface of the cornea becomes irregular and the vision becomes blurry. The resulting shape of the cornea is similar to a pregnant belly whereby the lower portion of the cornea protruding forward. PMD is often misdiagnosed as Keratoconus, although similar, the resulting cornea shape can be quite different. PMD often has cornea sizes similar to that of a regular eye but a very steep curve in the bottom of the cornea. PMD has been observed in families giving it an inherited trait and on occasion it has been observed unilaterally in one eye only. Typically it is bilateral affecting both eyes.
It is important to rule out both these conditions in LASIK candidates to avoid post-LASIK keratectasia which represents an iatrogenic exacerbation of the disease. Corneal topography is essential for the diagnosis of form fruste (preclinical) keratoconus where the patient may not recognize the symptoms, visual acuity may be good, and the cornea may appear normal on slit-lamp biomicroscopy.
Placido-ring corneal topography is usually represented as axial and tangential power maps. The recognition of keratoconus and PMD on these maps is not easy because there is no simple characteristic pattern. Early stages of the disease are especially difficult to distinguish from normal variations. The topography of central keratoconus shows central steepening which may be difficult to distinguish from the natural asphericity of the cornea. An off-center cone may produce an asymmetric bowtie, skewed bowtie, J, or U shaped patterns. PMD typically presents a “butterfly” pattern. As a result, diagnostic systems on the basis of the axial and tangential maps must employ multiple indices that extract features of these different patterns.
Therefore, there still exists a need for a better method of detecting and diagnosing ectatic diseases of the cornea.